From early days, attempts have been made to separate organic compounds as contained in mixtures by the membrane process. Virtually none of them, however, have matured into successful commercialization.
Although the procedural superiority which is inherent in the membrane separation process has found due recognition, the process itself has not been adopted for actual commercial applications. This is mainly because the development of membranes appropriate for the separation of organic compounds as from their mixtures has remained retarded.
The method most popularly adopted to date for the separation of organic compounds as contained in mixtures is distillation. This method has been substantially established from the technical point of view. Unfortunately, however, it has a disadvantage that it cannot be used advantageously for the separation of substances having mutually near boiling points, the separation of azeotropes, and the separation of substances unstable owing to thermal hysteresis. The recent sharp rise in the oil price has urged early development of energy-saving methods for the separation of organic compounds. As one of such methods, the membrane separation process is expected to meet the needs of the times.
To anticipate future exhaustion of the world's oil deposits, the development of substitute energy for oil is a matter requiring immediate attention. Among other promising resources, the biomass has a very bright prospect of being actually adopted as a successful substitute because it has an advantage that it utilizes the solar energy, grows through the process of reproduction, and produces no notable effect upon the natural environment.
The ethanol which is obtained by fermenting the material issuing from the biomass is in the form of an aqueous solution containing ethanol in a concentration of about 10%. For this solution to be used effectively as a substitute energy for oil, it must be treated so as to have its ethanol concentration heightened. When the conventional distillation process is adopted for the purpose of heightening the ethanol concentration of the solution, the energy to be consumed for converting the biomass into the energy of its final form amounts to a huge volume such that the energy obtained from the biomass may possibly be deprived of its value as a substitute energy for oil. In the technology for the development of biomass, the development of a concentration process more efficient than the distillation process constitutes one of the tasks to which the greatest significance is attached.
Various methods have heretofore been proposed for the purpose of obtaining from mixtures of organic compounds with water, particularly from a mixture of ethanol with water, concentrated ethanol through selective permeation of water.
For example, U.S. Pat. No. 2,953,502 discloses production of concentrated ethanol from a water-ethanol azeotrope by the pervaporation of the azeotrope by use of an acetyl cellulose membrane. This method is reported to have a separation factor of 8.5. For the separation of a water-ethanol azeotrope having a high ethanol concentration, this separation factor is low. Besides, the feasibility of this method is not sufficient because the membrane has much to be desired with respect to thermal resistance and chemical stability. In the Journal of Membrane Science, 1 (1976), pp. 271-287, there is reported a method for the concentration of a water-ethanol azeotrope by use of a membrane having poly(N-vinylpyrrolidone) grafted to polytetrafluoroethylene. In this case, however, the separation factor is 2.9, a value still lower than that obtained by the aforementioned method. Similarly to the membrane used in the aforementioned method, the membrane used in this method is deficient in separation capacity. Japanese Patent Publication No. 10548/1979 and No. 10549/1979 also teach methods for the separation of mixtures of organic compounds with water. These methods, however, lack feasibility in terms of the permeation rate.
From the disclosures such as of Japanese Patent Publication No. 41035/1976, Japanese Patent Publication No. 29988/1977, and U.S. Pat. No. 3,925,332, it has already been known to the art that hydrophilic membranes having an ion-exchange ability are obtained from ethylenic copolymer films by quick, uniform incorporation of a sulfonic group throughout the entire thickness of such films.
Further from the disclosure of U.S. Pat. No. 3,925,332, it has been known to the art that a hydrophilic membrane having an ion-exchange ability is similarly obtained from a film of a resinous composition comprising an ethylenic copolymer and a thermoplastic resin relatively inactive to the sulfonating agent.
These hydrophilic membranes have been developed as electroporous type membranes intended for uses as ion-exchange membranes, diaphragms in electrolytic cells, and membranes for dialysis. They are specific membranes in respect that, in addition to outstanding ion-exchange ability, they exhibit a high barrier property to anions, offer only a small degree of electric resistance in electrolytes, and retain the flexibility peculiar to ethylenic copolymers.
Although these hydrophilic membranes possess extremely high chemical stability in aqueous solutions of pH values ranging from the strong acidic zone through the neutral zone and the alkali zone, they have a disadvantage that they are gradually deteriorated by the action of oxidative chemicals and their various properties are accordingly degraded. The hydrophilic membranes have another disadvantage that since they assume large water contents in aqueous solutions and consequently exhibit high degrees of area swelling, they have weak strength in aqueous solutions. This particular disadvantage goes to restrict the usefulness of the hydrophilic membranes in proportion as their electric resistance in electrolytes is lowered and their thickness is decreased.
Concerning the use of the hydrophilic membrane obtained by incorporating a sulfonic group in an ethylenic copolymer as a diaphragm in a lead cell, there has been proposed in Japanese Publication of Unexamined Patent Application No. 140543/1978 a diaphragm for separating an anode and a cathode in the electrolytic cell, which diapharagm is obtained by using any of the hydrophilic membranes heretofore known in the art as described above and a varying reinforcing material such as, for example, a woven, knit, or non-woven fabric of inorganic fibers such as glass fibers, asbestos fibers, alumina fibers, or zirconia fibers which are bad conductors of electricity and are resistant to sulfuric acid or a woven, knit, or non-woven fabric or porous membrane of organic high molecular compounds such as polyethylene, polypropylene, rubber, nylon, polyester, or cellulose which is a bad conductor of electricity and is resistant to sulfuric acid.
The primary object of this diaphragm resides in precluding otherwise possible oxidative degradation of the hydrophilic membrane by preventing the hydrophilic membrane from being directly exposed to contact with the electrode. Further, owing to the physical combination of the hydrophilic membrane and the varying reinforcing material, the composite membrane so produced becomes easier to handle as during the assembly of a lead cell and serves advantageously as a diaphragm for the lead cell. In the production of this diaphragm, since the hydrophilic membrane to be used therein is prepared by one of the known methods such as of U.S. Pat. No. 3,925,332, it is extremely difficult for the hydrophilic membrane to be obtained in an extremely small thickness. Namely, when the membrane thickness is decreased to not more than 10 .mu.m, the strength of the membrane is so inferior in the sulfonation bath during or after sulfonation that the membrane is readily torn or broken. Therefore, it is difficult to sulfonate the membrane continuously and stably, and to remove the membrane from the sulfonation bath. This constitutes a serious problem in view of the commercial continuous production process. When the hydrophilic membrane to be used has a particularly small thickness, it exhibits poor resistance to oxidative degradation and the lead cell using this membrane offers a short life cycle. To meet the purpose of this particular invention, therefore, the hydrophilic membrane is desired to have a thickness in the neighborhood of about 40 .mu.m as indicated in the examples cited herein below.